1. Field of the Invention
The present invention relates to a punching device, a conveying device, a finishing device, and an image forming apparatus.
2. Description of the Related Art
For example, Japanese Patent Application Laid-open No. 2006-082936 discloses a conventional technology in which a punching device includes a sheet conveying unit that conveys a sheet and a punching unit that punches the sheet that has been conveyed by the conveying unit. The punching device also includes a branching unit, which branches a conveying path to a first and a second conveying paths, at a downstream of the punching unit. Japanese Patent Application Laid-open No. 2006-160518 discloses another conventional technology in which a punching device offsets a waiting position, which is in a direction perpendicular to a sheet conveying direction of a punching unit, by a predetermined distance from a position at which the punching unit performs a punching relative to the sheet, and the punching unit starts a punching preparation motion from the waiting position after a leading end of the sheet passes through.
In both the conventional technology, lateral registration is detected by a plurality of sensors, and a punching position is changed depending on the detection result to improve accuracy of the punching position.
Specifically, a side edge (lateral registration) of a sheet is detected by a charge coupled device (CCD) line sensor (lateral-registration detection sensor) to control movement of a punching unit depending on a misalignment amount of detected sheet edge to improve the accuracy of the punching position. At this time, the CCD line sensor may not detect an end surface due to a type of the sheet (e.g., a color and a difference in a reflection ratio of a printed image). In this case, especially when a print in black is performed, the end surface cannot be detected. This is generally regarded as an error and the process is terminated, or punching is performed even if the misalignment amount is in a range regarded as an error.